The invention relates to a method for directly screwing together at least two components using a hole-drilling and thread-cutting screw, the components being pressed against one another by means of a holding down clamp during the screwing-in operation.
Furthermore, the invention relates to a device for carrying out the method.
The term “directly screwing together” is understood to mean the production of a screw connection between at least two components to be joined without the previous introduction of holes or bores (what are known as pilot holes) into said components. In the case of directly screwing together, both the effort required producing the pilot holes and the effort required to find the holes during the alignment of the components to be joined are eliminated.
In the context of the invention, the direct screwing operation is to take place by way of a hole-drilling and thread-cutting screw. The screw has a head and a shank which is integrally formed on the head. The shank includes a self-tapping threaded section and a hole-drilling section for flow drilling. In the case of what is known as “flow drilling screw connecting,” the hole-drilling and thread-cutting screw, which is configured specifically for this purpose, is pressed with its hole-drilling section at the connecting point or screwing-in point against the components which are to be joined together, are aligned with respect to one another and are not pilot drilled, and is set in rotation. As a consequence of the friction which is produced, the connecting point is heated locally, which makes plasticization of the component materials possible, which are thereupon plastically deformed and finally penetrated in the radial and in the axial direction by way of the hole-drilling section. Here, what is known as a passage (also called a flow-drilled hole) is also formed on at least one of the components to be joined, into which passage subsequently the self-tapping threaded section of the hole-drilling and thread-cutting screw engages and produces a non-positive and positively locking connection between the components. Single-sided accessibility at the connecting point is advantageously sufficient.
A hole-drilling and thread-cutting screw for flow drilling is known from DE 39 09 725 C1.
In the case of directly screwing together, as explained in the preceding text, an undesired and disadvantageous gap formation can occur between the components on account of the plastic deformation of the component materials. In order to prevent a gap formation of this type, DE 103 48 427 A1 proposes the use of a holding down clamp which presses the components against one another during the screwing-in operation. The formation of a gap between the components is to be prevented by way of the holding down force which is active.
The invention is based on the object of specifying a method of the type mentioned at the outset which meets new requirements and which does not have, or at least has only to a reduced extent, the disadvantages which are associated with the prior art.
This and other objects are achieved by way of a method and device according to the invention.
The method according to the invention for directly screwing together at least two components without a pilot hole using a hole-drilling and thread-cutting screw which has a head and a shank which is integrally formed on the head with a self-tapping threaded section and a hole-drilling section for flow drilling, the components being pressed against one another by way of a holding down clamp (at the connecting point or screwing-in point) during the screwing-in operation, provides that the force (or holding down force), with which the holding down clamp presses the components against one another during the screwing-in operation, is varied during the screwing-in operation.
A “varying holding down force” is understood to mean that the holding down force, with which the holding down clamp presses or squeezes the components against one another during the screwing-in operation, is not constant, but rather changes over the screwing-in travel, which is effected, in particular, according to a defined or preset force-displacement profile.
It is the idea here that the holding down clamp does not press against the components during the entire screwing-in operation with the maximum required holding down force or pressing force for preventing a gap formation, but rather only at the correct time or during a comparatively short time period, as a result of which the components and, in particular, the upper component, on which the holding down clamp acts directly (or optionally also indirectly), are protected. As a result, in particular, components made from sensitive component materials or base materials can be protected and surface damage on the upper component can be prevented or at least minimized.
It is particularly preferably provided that the holding down force is highest or reaches its highest value (maximum) during the formation of a flow-drilled hole in the lower component, since the tendency for gap formation as a result of displaced component material is particularly high in this screwing-in phase. However, the holding down force can also reach its maximum, or a second maximum, toward the end of the screwing-in operation, as will be explained in greater detail in the following text.
The components to be screwed together are, in particular, individual parts or structural components for a motor vehicle body, such as metal panels or panel-like structures and profiles (which can be present as semifinished products or as three-dimensionally shaped components), and physical structures such as cast parts and the like.
It is preferably provided that the uppermost component, on which the holding down clamp is seated directly, is formed from a non-metallic material or substance, such as from a fiber-reinforced plastic (FRP) and, in particular, from a carbon fiber-reinforced plastic (CRP). In particular, the method according to the invention proves very advantageous for this purpose, since excessive temporary pressure loading on the upper component is avoided. It is preferably provided that the holding down force is maximum during the screwing-in operation only when this is absolutely necessary in order to prevent a gap formation and assumes a lower value during the other screwing-in phases (and can even assume the value zero, that is to say, no holding down force). Furthermore, damage to the upper component caused by the hole-drilling and thread-cutting screw, such as ripping out and chipping of material, can also be prevented or at least minimized in this case by way of a holding down clamp. The holding down clamp can therefore have multiple advantageous effects.
The holding down force can preferably be varied in the range from 0 N to 2000 N during the screwing-in operation. In the case of a holding down force of 0 N, the components are not pressed together at the connecting point. The holding down force can also assume a value of less than 0 N, as a result of which, for example, drawing back of the holding down clamp is made possible. It is particularly preferably provided that the holding down force is varied in the range from 100 N to 2000 N and, in particular, in the range from more than 1000 N to 2000 N.
Furthermore, it is preferably provided that the holding down clamp acts on the upper component in an, in particular, circularly annular area relatively tightly, in particular as tightly as technically possible, around the connecting point or the screwing-in point. It is particularly preferably provided here that the circularly annular area is as great as possible, with the result that the surface pressure which results from the holding down force which is applied is at a low level and damage to the components and, in particular, to the upper component (for example, pressing of the holding down clamp into the surface) is prevented or at least minimized. A great circularly annular area is distinguished, for example, by the fact that its radial annular width (radial spacing between the inner circle and the outer circle) corresponds to from 2 times to 10 times and, in particular, from 3 times to 7 times the head diameter of the hole-drilling and thread-cutting screw which is used. It is particularly preferably provided that the circularly annular area has an external diameter in the range from 20 mm to 60 mm, in particular depending on the head diameter of the screw. The contact area of the holding down clamp can also be configured with a different shape, that is to say such that it is not circularly annular. Furthermore, the holding down clamp can be configured on its contact face with a protective coating or the like for protection of the components and, in particular, the upper component.
It is preferably provided that the hole-drilling section of the hole-drilling and thread-cutting screw is configured specifically for the introduction of a hole into the upper component. The introduction of a hole into the upper component can be effected by way of forming or inward forming (that is to say, by way of plastic material displacement) and/or by way of cutting or incising (that is to say, material severing and/or by way of chip removal, rasping, rupturing and the like).
It is particularly preferably provided that the hole-drilling and thread-cutting screw which is used has at least one cutting edge or the like on its hole-drilling section, which cutting edge makes the introduction or production and, in particular, incision of a hole, in particular into the upper component, possible during rotation of the screw in one rotational direction and which permits the flow drilling, in particular in the lower component, during rotation of the screw in the other or in the opposite rotational direction. The at least one cutting edge on the hole-drilling section makes it possible, by way of rotation of the screw first of all in one rotational direction, to produce or form a hole with the removal of material or optionally also without the removal of material, in particular into the upper component, the holding down clamp having an assisting function here. By way of subsequent rotation of the screw in the opposite rotational direction, a flow-drilled hole can be formed with the same hole-drilling section, in particular in the lower component, into which flow-drilled hole subsequently the self-tapping threaded section can engage, as explained at the outset. In this way, in particular, components made from different materials (for example, FRP or CRP components and metal components, as will be explained in greater detail in the following text) can also be joined mechanically (composite construction).
The upper component can be formed from a non-metallic material or substance, preferably from a material which cannot be deformed plastically or can be deformed plastically only with difficulty, and, in particular, from a brittle material. The upper component is particularly preferably formed from a fiber-reinforced plastic (FRP) and, in particular, from a carbon fiber-reinforced plastic (CRP), as described above. The lower component can be formed from a metallic material and preferably from a plastically deformable or ductile component material. The lower component is particularly preferably formed from aluminum or steel.
The components to be screwed together can be additionally adhesively bonded to one another at least in the region of the screwing-in point. In precisely this operation, direct screwing together proves particularly advantageous, since special precautions are not required, such as keeping a pilot hole region free from adhesive, which leads to an interruption of the adhesive seam or adhesive area and therefore to strength and/or durability losses. Contamination of the production system or its tools by way of adhesive which escapes is also dispensed with. By way of the use of a holding down clamp, as proposed, the strength and/or resistance of the adhesive bond which is produced between the components can additionally be improved. It can be advantageous for this purpose that the holding down force is increased again or reaches its maximum toward the end of the screwing-in operation. As a result, what is known as breathing of the components with the formation of a gap can be prevented effectively.
The device according to the invention for carrying out the method according to the invention comprises at least:                a screwing spindle, by way of which a pressure force and a torque, with an associated rotation of the screw, can be applied to the hole-drilling and thread-cutting screw which is to be screwed in during the screwing-in operation; and        a holding down clamp which is configured in such a way that it can apply a variable holding down force on the upper component during the screwing-in operation.        
It is preferably provided that the device according to the invention has at least one adjustable or controllable actuating drive for the holding down clamp, which actuating drive serves to generate a holding down movement and/or a holding down force. It is preferably an electromechanical actuating drive. Instead of an electromechanical actuating drive, as an alternative or in addition, a hydraulic and/or pneumatic actuating drive can also be provided. It is provided, in particular, that said actuating drive can be controlled electronically or optionally also hydraulically or pneumatically by way of a control device which belongs to the device in order to set, control or change the holding down force during the screwing-in operation.
Furthermore, a regulation of the holding down force and/or the angular velocity or rotational speed of the screwing spindle can also be provided during the screwing-in operation. The holding down force can also be set or regulated during the screwing-in operation depending on the axially acting screwing force (this is the pressure force which is applied to the screw in the axial direction by the screwing spindle during the screwing-in operation), it being possible for the axially acting screwing force (also called bit force) to be variable and, in particular, controlled, regulated during the screwing-in operation itself.
Furthermore, it is preferably provided that the screwing spindle and, in particular, also its drive devices are designed or configured, in addition to the option for varying the rotational speed and/or the torque which is applied to the screw, for selective right-hand/left-hand running or for a reversal in rotational direction and, in particular, for rapidly changing right-hand/left-hand running. The rotational speed of the screwing spindle preferably lies in the range from 100 to 7000 revolutions per minute and can also be changed and/or regulated in said range.
In the following text, the invention will be explained in greater detail by way of example and in a manner which is not restrictive using a diagrammatic FIGURE which is not true to scale. Independently of specific feature combinations, the features which are shown in the FIGURE and/or are explained in the following text can be general features of the invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.